1. Field of the Invention
This invention relates to improved ferromagnetic fluid compositions commonly referred to as "ferrofluid compositions" in which fine particles of ferromagnetic material are dispersed in a very stable manner. More particularly, the present invention relates to ferrofluid compositions having low vapor pressure and low viscosity and which are suitable for use in seals under vacuum.
2. Prior Art
As conventional ferrofluids for a vacuum seal, there have been proposed two types of ferrofluids. One utilizes polyphenyl ether oil as its liquid carrier for dispersing therein the ferromagnetic particles, as disclosed by U.S. Pat. No. 4,315,827, and the other one utilizes alkylnaphthalene oil as disclosed by Japanese Laid-open (unexamined) Patent Publication No. Sho 59(1984)-168097.
Although the ferrofluid of the former is suitable for ultra low vacuum seals, due to its polyphenyl ether oil as a liquid carrier, having a very low vapor pressure of less than 10.sup.-7 torr, it, adversely, has high viscosity, since the viscosity of polyphenyl ether oil is 120 cst at 40.degree. C. This brings about high torque when the ferrofluid is used for a rotary shaft, and, thus, results in frictional heat within the ferrofluid itself, or at the peripheral machine parts or components to which the former ferrofluid is applied, thereby degrading the sealing power of the related machine parts.
On the other hand, as the latter utilizes alkylnaphthalene oil as its carrier, there exists no problem with respect to the viscosity. However, there arise other problems as explained below due to the fact that it uses petroleum sulfonic acid as a surfactant for dispersing fine ferromagnetic particles throughout the carrier.
More particularly, petroleum sulfonic acid has various portions of hydrophobic groups, among which there are contained some components which have poor affinity with the alkylnaphthalene oil carrier. Fine particles of ferromagnetic material which have adsorbed these components having poor affinity, naturally become poor in dispersion property and are liable to precipitate or settle within the carrier, thereby decreasing the yield in producing the same, and, further, it becomes impossible to obtain a ferrofluid in high concentration.